This invention relates to making and using an aqueous slurry of hydraulic cement. More particularly, it relates to providing such a slurry which contains nitrogen gas-forming reactants having concentrations and reaction rates which are correlated with the temperature and pressure in the zone to be cemented to provide a volume-stabilizing amount of gas production during substantially all of the setting and hardening of the cement.
Numerous patents and publications have appeared regarding foamed cements and their preparation and use. Such cements have been said to be useful as lightweight cements, construction materials, grouting materials, thermal insulating materials, non-shrinking cements, gas-impervious cements, fluid-permeable cements, and the like. The procedures for making and using such foamed or non-shrinking cements have included modifying conventional cement slurries by adding various hydrogen generating reactants, nitrogen generating reactants of numerous kinds, oxygen-generating reactants, water-swellable materials, and the like.
U.S. Pat. No. 2,163,207 relates to making cellular building materials by combining lime base binders with aqueous ammonia and hypochlorites (and optionally, peroxides) to produce gaseous nitrogen.
U.S. Pat. No. 2,191,555 relates to making porous cementous building material by adding to non-alkaline formulations (such as calcined gypsum plasters) an amide (such as urea) and a material which forms nitrous acid (such as a nitrite salt).
U.S. Pat. No. 2,288,556 relates to making a permeable cement by adding to a hydraulic cement slurry enough gas-generating material (such as powdered aluminum, calcium carbide, ammonium nitrite, or the like) to form a network of interconnected gas bubbles.
U.S. Pat. No. 2,371,928 relates to making porous materials from slurries, of cement gypsum or the like, by first mixing the cement with water, hypochlorite and filling material, then adding a pore-stabilizing substance (such as a soap or glue) along with hydrogen peroxide to generate gaseous oxygen.
U.S. Pat. No. 3,420,299 relates to preventing shrinkage during the setting of a cement slurry by adding pellets of a water-swelling material (such as bentonite) encapsulated in a material (such as a gel-like mud) to be ruptured when the cement slurry begins to harden.
U.S. Pat. No. 3,591,394 relates to making porous cement by adding to the cement slurry a "nitrogen delivering" compound (such as a diazonium compound) and an activator (such as sodium aluminate, potassium sulphate lead peroxide, sodium perbolate or the like) and mentions the desirability of delaying the onset of gas production until the slurry has been pumped into place.
U.S. Pat. No. 3,926,257 relates to making a foamed cement which is impermeable to gas by adding a foaming surfactant to the cement slurry so that any inflowing gas will form an immobile foam within the setting cement.
U.S. Pat. No. 4,142,909 relates to making a non-shrinking cement by adding to a hydraulic cement slurry a gas-generating compound which is capable of a controlled reaction without activators (such as azodicarbonamide, sodium azodicarboxylate, an organic peroxide, or sodium borohydride).
Numerous technical journal publications relate to the problems caused by gas leaking through a cemented annulus of a well borehole.
An article G. Carter and K. Slagle (of Halliburton) "A Study of Completion Practices to Minimize Gas Communication", September, 1972, JPT, page 1170, indicated that the problem was first recognized as being significant in the middle 1960s and two important factors for preventing the leakage are the maintenance of both a hydrostatic head greater than the gas pressure and a low fluid loss within the cement slurry.
An article by W. W. Christian, J. Chatterji and G. W. Ostroot of Halliburton, "Gas Leakage in Primary Cementing--A Field Study and Laboratory Investigation", November 1976, JPT, page 1361, mentioned that with deeper well completions across gas-producing horizons the problem of gas leakage was then a major concern and concluded that the lack of fluid loss control may be a primary cause of such leakage.
SPE Paper 8255, September 1979, "Annular Gas Flow After Cementing: A Look at Practical Solutions" (by representatives of Exxon and Texas A&M) concluded that a hydraulic pressure loss during the hydration reaction within the cement slurry is a primary cause of such leakage and the means for preventing it include cementing in short stages with slurries having different curing rates or using a foamed slurry containing gas bubbles that act as compressible pressure compensators during the hydration reaction.
SPE Paper 8257, September 1979, "Study of Factors Causing Annular Gas Flow Following Primary Cementing" (by representatives of Halliburton) indicated that the problems due to the pressure loss during the cement hydration can be removed by "an entrainment, addition, or in situ generation of a gas".
SPE Paper 8259, September 1979, "Flow After Cementing-A Field and Laboratory Study" (by representatives of Mobil and B-J Hughes) indicated that a cement slurry free water content of any significant magnitude is a significant factor (particularly in inclined boreholdes) capable of causing a gas flow after cementing.
Thus, the relatively recent journal publications indicate the desirability of having gas bubbles distributed uniformly within a cement slurry at the time the slurry is losing its ability to provide a hydrostatic pressure exceeding the pressure in an adjoining gas-containing reservoir. And, the issued patents show that it has long been known that numerous reactive compounds and procedures are available for causing numerous kinds of gas bubbles to be present within a cement slurry.
However, it is also known that (1) too little gas will provide insufficient compensation for the pressure loss during the hydration of the cement, (2) too much gas will cause the cement to have too much permeability and/or too little strength, (3) too early an inclusion of gas within the cement slurry will allow the gas to become non-uniformly distributed with respect to the other components of the slurry (which are much heavier and/or much more viscous than the gas), (4) too late an inclusion of the gas will fail to keep the reservoir gas from entering and channeling through the hydrating slurry before the cement has set and (5) the inclusion of a highly explosive gas such as hydrogen or a combustion-inducing gas such as oxygen will create a chemical situation which is difficult or dangerous to handle. And thus, the teachings of prior patents and publications are clear that a method for causing some sort of gas formation is desirable--but also show that no such method which is free of disadvantages that may be difficult or impossible to overcome has yet been disclosed.